Listener Crossword 4153: Rhythm for a Dance by Brimstone

[Zabadak]

I think I have it, though one element, the two letter replacement, and one additional word entry are as yet unconfirmed. I'm still trying to justify everything, which looks like being a bit of a slog, but it all looks as if it fits. I have the characters and the title, and I'm half inclined to say "enough is enough" just now, and look on it all with fresh eyes tomorrow.
At the moment, it all feels a bit messy, but thanks to Brimstone for a considerable workout.

[serpentinew]

Contendo - I am aware of the tradition here and don't intend to breach the rules. As neither word is the answer, I did n't feel I had transgressed as this led to the unmodified answer. In neither case does my information assist in entering the modified answer, but it does help tie up a loose end for Jack IMO.

[ichkeria]

Someone suggested this theme has not been done before but it certainly has, a few years ago

[easterfool]

Yet again, one of the last to finish. Knew that I had everything right but not always sure why. Don't like that feeling, so struggled on with a bit of help from a friend. Now I can fill and send in with a sense of completion. Thanks for the comments that said that others had been in the same quandary - it makes me feel normal. Is that a viable comment on Listener Crossword solvers?
Thanks Brimstone - was this your first? If so, I will look froward to the next - and start it as early as possible!